1. Field of the Invention
The invention relates to a method for computer assisted determination of positioning parameters for positioning of a medical device relative to a target structure and to a medical device for use with the method.
The task of placing implants plays a key role in trauma and orthopedics as well as in several other medical disciplines. Not only with upcoming minimal invasive techniques, orientation in anatomical terrain is demanding and requires experience of the operator. Prolonged surgery time accompanied by intense radiation and poor surgical results are the consequence. Computer assisted surgery (CAS) concepts aim at easing surgical handling. They are generally based on a three step approach where in a first step medical imaging of the region of interest is required, secondly a virtual surgical plan of the operation is generated and in a third step the surgeon is reliably guided in executing the plan on the patient. Medical imaging is mainly performed by CT (Computed tomography), MRI (Magnetic resonance imaging) or radiography such as 3D isocentric scanning. On the costs of a considerable radiation dose, these technologies deliver highly defined 3D images for convenient planning but confront particularly the smaller clinics with a major investment. Moreover, in many hospitals CT or MRI are considered as a bottleneck in the clinical workflow. After the surgical planning costly and accident-sensitive tracking devices based on optical, mechanical, electromagnetic or acoustic principles are required in the operation room to interlink the patient with the operational plan. This step is called patient registration which is currently one of the most challenging issues in the field of computer aided surgery and influences significantly the overall accuracy of the surgical procedure. For tracking of anatomy and implants, reference bases need to be rigidly and in many cases invasively attached to several relevant elements. This is generally perceived as a time consuming and hindering necessity during surgery. Hence, a simplified assistance technology for reliably and efficiently performing medical interventions carries potential to significantly improve current surgical workflows.
2. Description of the Related Art
A computer assisted surgery system and a method for operating the same is known from US-A 2011/213379 BLAU ET AL. This known computer assisted surgery system essentially comprises a reference body which can be positioned in relation to an anatomical structure, a detector device for detecting the position of the reference body in relation to the anatomical structure and a computer suitably programmed for superimposing the anatomical structure with a virtual representation of a medical device, e.g an intramedullary nail, to modify the position of the reference body and to optimize the virtual position of the medical device, e.g. the intramedullary nail with respect to the anatomical structure.
The reference body can be fixed to a medical tool, e.g. a bore tool or to an intramedullary nail. The reference body is provided with a plurality of fiducial markers which are distributed over the reference body so that the position of the reference body can be determined from a single two-dimensional medical image. Alternatively, the reference body can be a medical tool having a unique geometry to identify the position thereof in a medical image. In another alternative embodiment an intramedullary nail can be the reference body because the intramedullary nail may have a unique form.
The method for operating the above computer assisted surgery system comprises the steps of:                a) positioning of a reference body in relation to an anatomical structure;        b) taking a first 2-dimensional medical image at a first angle, the reference body virtually representing a position of a medical device, e.g. an intramedullary nail to be applied to the anatomical structure;        c) viewing the image data obtained from the first 2-dimensional image on a display device;        d) viewing the anatomical structure with a virtual representation of a medical device, e.g. an intramedullary nail to be applied which is superimposed on the image data;        e) viewing an optimal virtual position of the medical device, e.g. the intramedullary nail to be applied to the anatomical structure so as to obtain a best fit with respect to rules, e.g. a surgical plan for allowable ranges for applying the medical device, e.g. the intramedullary nail to the anatomical structure; and        f) modifying the position of the reference body.        
One problem associated with the above described computer assisted surgery system and the method of using the same can be that the position of an implant or a surgical instrument is determined on the basis of a separate reference body comprising a plurality of markers which has to be attached to the implant or instrument in a defined position.
Furthermore, fiducial markers need a comparatively wide spreading for adequate tracking. This can be problematic since the field of view of a conventional C-arm is limited and the space for attachment of the markers is restricted. Furthermore, undesired overlapping in the projected medical image might occur. This wide spreading of the markers or targets is not required when using cylindrical reference means. Radiopaque markers must be embedded into a radiolucent structure like a carbon handle. The fiducial markers cannot be used e.g. on metallic implants. Producing radiolucent means equipped with radiopaque markers is more demanding than drilling holes into existing structures or even using existing holes.
Thus, there remains a need for an improved method and device for computer assisted surgery.